Four feet below the streets of major American cities like Boston and Chicago lie tens of thousands of miles of cast iron and steel pipelines dating back to the end of the 19th century. Originally built to transport "town gas," an early coal-based heating and cooking fuel, the same pipelines are now used to distribute natural gas (methane) to millions of homes.

The problem is that many of those century-old pipelines are heavily corroded, creating widespread methane leaks in urban areas. Methane is a potent greenhouse gas, packing more than 80 times the warming power of carbon dioxide over a 20-year period. Utilities are aware of the problem, but replacing just a mile of natural gas pipeline costs between $1.5 and $2 million. To save money, utilities fix the pipes that pose an immediate safety hazard and let slower leaks persist for decades.

Now an innovative collaboration between environmentalists, scientists, and Google is providing utilities with a new tool to combat the most polluting methane leaks. Researchers at Colorado State University have equipped Google Street View cars with special methane sensors that can not only map the precise location of leaks, but also gauge their severity.

Joseph von Fischer is a biology professor at Colorado State and lead author of a recent paper in Environmental Science and Technology about the laser-based sensors that turned a fleet of Street View cars into methane-sniffers. The project, which has mapped gas leaks in cities like Boston, Chicago, and Los Angeles starting in 2013, is funded entirely by the Environmental Defense Fund (EDF).

In Boston, where more than half of the city's pipes are more than 50 years old, Google's Street View cars found an average of one methane leak per mile of pipeline. In Chicago, where 37 percent of pipelines serviced by utility Peoples Gas were made of cast iron or other corrosive materials, the mobile sensors detected one leak for every three miles. In Indianapolis, where a deadly explosion from a gas leak prompted extensive replacement of old pipes in the 1980s, there was only one leak for every 200 miles of pipeline.

"We don't have a good numbers about how much methane emissions from cities contribute to the overall US anthropogenic natural gas footprint," von Fischer told Seeker, explaining that emissions from natural gas production sites are undoubtedly larger, but harder to track and fix. "One of the reasons we're focusing on methane leaks in cities is that we can actually do something about them."

New Jersey is an early success story. The state's largest utility, Public Service Electric & Gas (PSEG), partnered with EDF and Google in 2016 to map hundreds of miles of urban pipeline over six months. The utility had budgeted $905 million for a three-year pipe replacement project, targeting specific stretches of pipeline based on its own methane-sensing data. When the Google cars finished collecting their data, including a ranking of leaks from worst to least flow, PSEG saw that its original plan was way off.

Armed with a detailed methane leak map from von Fischer's team, PSEG came up with a new replacement plan that will reduce overall methane emissions by 83 percent while requiring 35 percent fewer miles of replaced pipeline. The New Jersey utility is on track to swap out 510 miles of old pipeline by 2018.

The methane-mapping partnership was the brainchild of the Environmental Defense Fund and Google Earth Outreach, a branch of the search giant that helps nonprofits tap the company's extensive mapping tools and data. Von Fischer and his team at Colorado State were brought in to work out the sensing technology.

Google Street View cars are famous for the bulky 360-degree cameras mounted on their rooftops, but that's not where the laser-based sensor lives. The system sucks in city air from an opening in the car's front bumper and pumps it into a small tube in the trunk. The air sample is then blasted with a pulse of infrared light. Since greenhouse gases like methane absorb infrared light, you can measure the amount of methane in the sample by measuring the amount of laser light that escapes the tube.

Onboard computers use a series of algorithms match the continuous flood of data - 2,000 data points per minute - with precise GPS locations to generate the leak map. To avoid false spikes produced by street-level emissions from natural gas-powered buses, the Street View drivers were instructed to drive each route multiple times. Elevated methane readings were only included if they showed up two or more times in the same location.

The methane-mapping project isn't the only research effort using Google Street View cars as mobile monitoring stations.

Aclima is a San Francisco-based company that develops indoor and outdoor air-quality monitoring solutions. In 2014, Aclima partnered with Google Earth Outreach and the US Environmental Protection Agency to measure hyper-local levels of greenhouse gases in Denver. A pilot team of three Street View cars equipped with Aclima's sensors swarmed Denver's streets for 750 hours and collected massive amounts of ground-level data on not only methane levels, but carbon dioxide, carbon monoxide, ozone, nitric oxide, and particulate matter.

Now Aclima and Google have brought the partnership back to California, where they're collecting data in three cities with the country's worst air quality: Los Angeles, San Francisco, and the Central Valley. Jessica Lass, communications director for Aclima, said in an email that they've driven 75,000 miles to date and will share maps and insight later this year.

Aclima believes that by getting neighborhood-by-neighborhood air quality data into the hands of both the public and policymakers, we'll all start to make smarter decisions about energy and transportation, and their effect on the environment and public health.

As von Fischer noted, urban methane emissions are only one part of the bigger story about air pollution across the natural gas production cycle. At the end of 2015, a massive gas leak at a natural gas storage facility outside of Los Angeles released 90,000 metric tons of methane into the atmosphere over the course of four months. That's the emissions equivalent of more than a million cars driving for a full year.